You need to be able to explain the processes that cause earthquakes. This means that you need to be able to explain how and why earthquakes occur.
Earthquakes happen every day in New Zealand. There are around 14,000 earthquakes in and around the country each year. Most are too small to be noticed, but between 150 and 200 are big enough to be felt.
The earthquakes are caused by the movement of the earths crust which is broken into sections called plates. These plates float on the liquid rock of the Mantle. The plates are moved by convection currents in the mantle, which circulate round forcing them to move towards and away from each other. They move at a very slow rate, normally just a few centimeters each year. About the same rate as finger nails grow.
Earthquakes are most common in areas where two plates meet. These areas are called plate boundaries. New Zealand lies on the plate boundary between the Pacific Plate and the Australian Plate.
When two plates are forced together, or past one another, they sometimes get stuck and stress energy builds as they try to keep moving. When they finally move this energy is released and an earthquake occurs. The energy is released in the form of seismic waves which travel through the earths surface. Earthquakes normally take place at faults which are existing cracks or fissures in the earths crust.
The strength of the earthquake depends on the size of the movement and the depth of the movement. The deeper beneath the earths surface that the actual movement occurs (the focus = point at which the earthquake or movement actually occurred) the less it will be felt at the surface (the epicenter = point on the earths surface directly above the focus). The shallow earthquakes, ones where the movement is nearer the surface, are they ones that tend to cause the severe damage.
